1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus and, more particularly to a width directional positioning of a sheet when the sheet of which one side an image is formed on is reversed, and conveyed to an image forming portion again to form an image on the backside of the sheet.
2. Description of the Related Art
Conventionally, in image forming apparatuses such as electrophotographic printers, there has been the one, which reverses a sheet on which an image has been formed on the first side, conveys the sheet to the image forming portion again, and thereby forms an image on the second side of the sheet. Furthermore, in an image forming apparatus capable of forming an image on both sides of a sheet like this, there is provided a sheet conveying apparatus in which a sheet on which one side an image has been formed on one side is reversed and conveyed to an image forming portion again.
Here, in such a conventional sheet conveying apparatus, in the case where a sheet is conveyed to an image forming portion again in order to form an image on the second side, the sheet may be skew-fed (conveyed in an oblique state), or fed while deviated in a width direction (in a direction orthogonal to a sheet conveying direction). Due to this skew feed or positional deviation of a sheet, when an image is formed on the second side, the position of an image with respect to the sheet is deviated. The cause of such a skew feed or positional deviation is that in the case of forming an image on the second side, a conveying path comes to be longer as compared with the case on the first side, thus being largely affected by the eccentricity or the difference in an applied pressure of each of a plurality of rollers disposed in a conveying direction for conveying the sheet, the difference in resistance on a conveying surface or the like.
Then, to prevent such a deviation of a sheet, after an image has been formed on the first side, the position of the sheet needs to be adjusted so that the sheet is registered with respect to an image on the way of conveyance until the image is formed on the second side. Thus, as such a sheet position adjusting method, there is a method which disposes a reference guide at one end portion of a re-conveying path conveying the sheet of which the first side an image has been formed on, and conveys the sheet while pressing the sheet against this reference guide, thereby making the positioning in a width direction of the sheet (lateral registration correction).
FIG. 8 is a view taken from above illustrating the construction of a sheet conveying apparatus making the positioning in a width direction (lateral registration correction) of a sheet S by such a so-called one-side reference.
With reference to FIG. 8, a reference guide 12 is disposed at one end portion of a re-conveying path 18. By pressing one side edge parallel with a sheet conveying direction of the sheet S against a reference surface 12a, 12b of this reference guide 12, the position of a side edge of the sheet S is adjusted. The end portion of the sheet S is pressed against one of the reference surfaces 12a and 12b for positioning, depending on the size of a sheet. A conveying lower guide 20 forms a lower surface of the re-conveying path 18 along with the reference guide 12.
Roller shafts 13 of two oblique-feed rollers 11a are held rotatably with respect to the reference guide 12 by bearings respectively. Further, respective pulleys 15 are fixed to the end portions of two roller shafts 13, and these two roller shafts 13 are driven by belts 16a and 16b passed over these pulleys 15. One belt 16a of these belts 16a and 16b is driven by a driving motor (not shown), and thus the oblique-feed rollers 11a are driven by this driving motor.
Moreover, a pin 17 rotatably holds an oblique-feed driven rotatable member 11b, as well as being supported by a conveying upper guide 19 illustrated in the below-described FIG. 1 that forms an upper surface (top surface) of the re-conveying path 18. A spring 181 presses the pin 17 from above. By the action of this spring 181, the oblique-feed driven rotatable member 11b is brought into pressure contact with the oblique-feed roller 11a under a predetermined pressure.
Furthermore, in the re-conveying path 18 adjusting the side edge position of a sheet S by the one-side reference, a sheet S of which the first side an image is formed on, and thereafter the sheet S being conveyed from the direction indicated by an arrow A is oblique-fed by the oblique-feed rollers 11a and the oblique-feed driven rotatable members 11b. 
Here, these oblique-feed driven rotatable members are oblique at a predetermined angle respectively so as to apply a conveying force toward a reference surface 12a, 12b to a sheet S in order to abut the sheet S on the reference surface. Whereby, the sheet S is conveyed while changing the orientation of the sheet S to be directed toward the reference surface side, and thus an end portion thereof is pressed against the reference surface 12a, 12b, thereby performing a positioning of the sheet S.
Incidentally, owing to a longer conveying path as described already, the eccentricity or the difference in an applied pressure of various rollers, the difference in resistance of a conveying surface or the like, there are some cases where the skew feed of a sheet S occurs until the sheet is conveyed to the re-conveying path 18. Also, a curl may occur at an end portion of the sheet after an image fixation.
Thus, conventionally, for example, as illustrated in FIG. 8, reference guide introducing portions 12c, 12d having a rake angle are formed at the reference surface 12a, 12b. Further, there is the one in which a sheet having been skew-fed is rotated in a thrust direction by a rake angle of these reference guide introducing portions 12c and 12d. This art is disclosed in Japanese Patent Application Laid-Open No. 2004-299856.
Here, as illustrated in FIG. 8, in the case where a sheet S is conveyed being deviated by X1 in the direction indicated by an arrow X, which is a thrust direction from the reference surface 12a, first a leading edge of this sheet S, as illustrated in FIG. 9A, is contacted a side end surface of the reference guide introducing portion 12c having a rake angle.
Here, since there is no conveying roller nip in the upstream vicinity of the reference guide introducing portion 12c, the sheet leading edge can rotate in the thrust direction. Whereby, thereafter, when conveyed in the direction indicated by an arrow B, the sheet S, as illustrated in FIG. 9B, due to the rigidity (stiffness) of a sheet leading edge, the sheet leading edge is slid along the reference guide introducing portion 12c without buckling, and then the leading edge of the sheet S is rotated in the Rx direction. Then, the leading edge of the sheet S is conveyed along the reference guide introducing portion 12c. 
Subsequently, the sheet S having been rotated in the Rx direction like this, thereafter, as illustrated in FIG. 9C, by the action of the oblique-feed roller 11a and the oblique-feed driven rotatable member 11b, is pressed against the reference surface 12, and fed to the image forming portion again while being positioned.
Like this, a sheet S having been conveyed being deviated by X1, is raked X1 by the reference guide introducing portion 12c to come close to the reference surface 12a while being rotated in the Rx direction, and thereafter fed to the image forming portion again while being positioned by the action of the oblique-feed roller 11a and the oblique-feed driven rotatable member 11b. 
By the way, in such a conventional sheet conveying apparatus and image forming apparatus, in order that a sheet leading edge can be rotated in the Rx direction, as illustrated in FIG. 10, there is formed a gap GA between a bottom surface on the opposite side to the reference guide 12 of the conveying lower guide 20 and a sheet S. Whereby, when the leading edge of the sheet S is made to come close to the central side by degrees, and thus the sheet is twisted with the reference guide introducing portion 12c, a portion on the opposite side to the reference surface 12a of the sheet S can be flexed in the gap GA, thus enabling the sheet S to rotate in the Rx direction.
When, however, a deviation amount X1 of a sheet S becomes larger, the amount of flexure of the sheet S when the sheet S is twisted gets larger, resulting in a higher rigidity of the sheet. Therefore, the sheet cannot be sufficiently flexed in the gap GA, and thus the rotation of the sheet S may be limited. In this case, a sheet leading edge on the reference guide introducing portion 12c side, as illustrated in FIG. 11, will be buckled on the side end surface of the sheet guiding portion 12c. As a result, a sheet S cannot be rotated in the Rx direction, and thus a skew feed cannot be sufficiently corrected or a buckled sheet end portion comes to be a conveying resistance, resulting in the occurrence of a sheet jam. In particular, the higher the rigidity of a sheet S is, the higher the rigidity when the sheet is twisted is, so that a buckling of the sheet leading edge on the reference guide introducing portion 12c side is likely to occur.
Like this, when a deviation amount X1 of a sheet S gets larger, when a sheet leading edge is contacted with a side end surface of the reference guide introducing portion 12c, the sheet leading edge is buckled, resulting in the occurrence of a sheet jam. Accordingly, when a deviation amount X1 in a thrust direction of the sheet S is raked up to the reference guide 102 with the reference guide introducing portion 12c, the rigidity of the sheet S needs to be not more than such a magnitude as allows the sheet to sufficiently flexed at a place of the gap GA.
On the other hand, in the case of a thin sheet S, or in the case of a low rigidity of the sheet S as is under high-humidity environments, when a deviation amount X1 of the sheet S gets larger, upon a sheet leading edge being contacted with the reference guide introducing portion 12c, the sheet leading edge is likely to buckle as illustrated in FIG. 11. As a result, the sheet S cannot be rotated in the Rx direction, and further the sheet end portion having been buckled becomes a conveying resistance, resulting in the occurrence of a sheet jam.
Furthermore, with a sheet having been curled at a leading edge portion after fixing, since the sheet is further curled when the sheet leading edge is contacted with a side end surface of the sheet guiding portion 12C, resultingly there will be a corner folding at the leading edge of the curled sheet, and thus a sheet jam is likely to occur.
That is, depending on the magnitude of rigidity of a sheet S or due to a curl occurring at the sheet S, when being conveyed along the reference guide introducing portion 12c, the sheet cannot change the direction thereof toward the reference guide, and thus a sheet leading edge portion may be buckled. Then, when the sheet leading edge portion is buckled like this, this buckled portion comes to be a conveying resistance, resulting in the occurrence of a sheet jam in the vicinity of the reference guide introducing portion.